This invention relates to current detector/indicators and in particular to improved current detector/indicators such as can be utilized by modern control systems as a means to facilitate rapid fault assessment and status of loop circuit integrity while providing circuit isolation. In other words, it is aimed at testing electrical contacts for mechanical and electrical function without breaking power to external circuits. Many uses are envisaged for this device but in particular there is the ability to supply visible information on whether a current is flowing in a circuit or not and to supply an isolated switched output of that information.
Light emitting diodes are excellent devices to indicate current flow but are generally limited in their current capacity to a maximum of 100 mA and are very prone to damage from reverse voltages of low magnitude. They exhibit extremely long life if subject to their design ratings of current and voltage and consume little. In this specification, it should be understood that as well as xe2x80x98ordinaryxe2x80x99 LEDs, such as are used in many indicators, it would also be possible to use laser diode type LEDs.
According to the present invention there is provided a current indicator comprising a transistor and an LED in circuit therewith and arranged to have a controlling influence thereon, current flow through the LED energising it and thereby causing the transistor to conduct and shunt excess current away from the LED and through the transistor.
In a preferred, simple form for indication of DC or one polarity of AC, there is one transistor and the LED is connected across its collector and base. The LED can be part of an opto-isolator capable of providing a remote indication of current flow, in which case another LED in circuit with the transistor might be arranged to be energised when the transistor is conducting, thereby to provide local indication of current flow. This other LED can be connected across the emitter and collector.
For indicating both polarities of AC, the indicator can be doubled with two transistors connected in inverse parallel.
In another convenient form the transistor is a triac in an AC power line which may pass current selectively full wave or either half wave, and wherein there are two LEDs, in parallel with reversed polarities, in the triac""s gate circuit arranged to trigger the triac into conduction when power is on the line, each LED being energized with the gate trigger current during the associated half wave conduction through the triac but extinguishing when the triac avalanches into conduction.
Again, each LED can be part of an opto-isolator capable of providing a remote indication of current flow, in which case another LED in series with each first LED may provide local indication of current flow.
Whenever there are two LEDs for visible indication of current they may be combined into a bi-color LED, AC being indicated by a mixture of the colors. More usefully, four different conditions may be delivered via two wires by way of two switches isolated by two inversely polarised diodes.
PNP and NPN transistors are usable. Where the circuit is doubled, two transistors of the same type may be used, or complementary PNP and NPN transistors.
Preferably, the transistor(s) will have reverse voltage protection, for example by a diode across the collector and emitter.
In one arrangement, a load may be energized through a power diode, across which is connected the emitter and base of a transistor, the collector being connected through the LED to the other side of the load. There could be two LEDs in parallel or series, one being for local indication and the other being part of an opto-isolator. It is important that the power diode and the transistor share a common thermal environment for reliable operation.